Some low-lying areas have been evacuated as a strong upper trough brought heavy rain and gale force winds to New South Wales, with the weather bureau warning conditions are expected to worsen with flooding, abnormally high tides and damaging surf.

Authorities have warned the weather is expected to become more severe into Sunday as an east coast low trough moves down from the north coast past Sydney and onwards to the south coast.

A flood watch has been issued for 23 NSW rivers.

Residents in Billinudgel, Ocean Shores, Golden Beach, New Brighton and in low-lying areas near Coffs Creek, on the western edge of Coffs Harbour, have been ordered to evacuate due to rising floodwaters.

The NSW SES warned people to get out immediately because staying means they could become trapped and need rescue.

At 6:00pm on Saturday, flood evacuation warnings were also issued for Chinderah, the Seagulls Estate, South Murwillumbah, the township of Tumbulgum and Fingal Point area, near Tweed, west of the single-lane temporary bridge.

The warnings said residents should prepare to evacuate if authorities alert them to leave.

Numerous trees also came down over roads around Sydney, including in Paddington and Petersham, and an apartment building in Vaucluse has had its roof blown off.

Several Vivid Sydney installations were damaged by the strong winds and organisers have urged visitors to delay their trip, with many installations to be switched off for the evening.

NSW SES commissioner Phil Campbell said the service had more than 2,600 calls for assistance across the state so far.

"With torrential rain and damaging to destructive winds still occurring, those calls are still coming in at a rate of around 50 to 100 per hour," he said.

A pedestrian was killed at Marks Point in the Hunter region after being hit by a car and a woman in her 20s was killed after the car she was driving crashed into a tree at Bulli in the Illawarra.

It is unclear if the fatalities happened due to the weather, however authorities are urging people to stay off the road unless the travel is essential.

20,000 properties without power

At 8:00pm, about 20,000 properties on the Endeavour Energy, Essential Energy and Ausgrid networks in Sydney and across the state were without power as trees had brought down wires and snapped power poles.

The storm has affected the Northern Rivers, Mid North Coast, Hunter, Metropolitan, Illawarra, South Coast, Central Tablelands, Northern Tablelands and parts of the Southern Tablelands and North West Slopes and Plains districts, the weather bureau said.

Residents in the Illawarra and south coast are bracing for more rain, after being hit with 100 millimetres on Friday night, as the east coast low continues to move south across the state.

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